1. Field of the Invention
The present invention relates to a conductive polymer composition exhibiting positive temperature coefficient (PTC) behavior. More particularly, it relates to a PTC composition which has intense, reproducible PTC behavior. The PTC composition is particularly useful in PTC thermistors.
2. Description of the Related Arts
PTC conductive polymer compositions are known for use in circuit protection devices and self-limiting or self-regulating heaters.
In the conventional art, PTC compositions are prepared by cross-linking a polymer component thereof by radiation or by chemical cross-linkers. For instance, Polym. Eng. Sci. 44, 532 (1973) shows a method of producing a PTC composition by blending high-density polyethylene (HDPE) and carbon black, followed by curing the HDPE with a peroxide initiator. However, such a method is high-cost and there are chemical residues which may corrode metal electrodes. Further more, the cross-linked polymer has inferior moldability and less intense PTC behavior owing to the higher degree of crosslinking density and the poor degree of crystallinity.
U.S. Pat. No. 5,545,679 discloses a PTC conductive polymer composition which uses thermosetting polyester resin combined with conductive fillers. However, the compositions using thermosetting resins as a polymer matrix have the drawbacks of being poor in PTC intensity and moldability.
U.S. Pat. No. 4,591,700 teaches a PTC conductive polymer composition which contains a mixture of two crystalline polymers of different melting points, where the melting point of one polymer is at least 160.degree. C. and at least 25.degree. C. higher than the melting point of the other polymer. In such a composition, the conductive particles are liable to segregate in the fusion state, thereby resulting in negative temperature coefficient (NTC) behavior. Moreover, the conductive particles are prone to segregate at the grain boundaries when the polymer is being crystallized, thus making the PTC behavior of the composition irreproducible.
In addition, the PTC polymer compositions of the prior art have generally failed to provide satisfactory adhesion to metal electrodes or to percolate a large quantity of carbon black and/or inorganic fillers.